Apertured diaphragms may be disposed between ion guides for differential pumping systems with low gas conductivity. These apertured diaphragms are implemented to restrict a cross-section of an ion beam, or to accelerate or focus ions directed between the ion guides.
Typically, metallically conductive diaphragms cause the RF fields between the ion guides to break down. The RF fields continue uninterrupted through the material of the high-resistance apertured diaphragms to the next ion guide, and therefore the otherwise usual ion losses at the transitions between identical ion guides no longer occur.
Here, mass spectrometry is defined as ion spectrometry. The term “mass” refers to a “charge-related mass” m/z, rather than a “physical mass” m. “z” is defined as the number of excess elementary charges of an ion. Therefore, in the following disclosure the terms “mass” or “mass of the ions” are to be understood as a charge-related mass m/z, unless otherwise indicated.
In modern mass spectrometers, RF ion guides are used to guide ions from an ion source to a mass analyzer, and to manipulate the ions, for example, for fragmentation by collisions. In the simplest case, the RF ion guides are configured as multipole rod systems; i.e., quadrupole, hexapole or octopole rod systems having four, six or eight parallel pole rods disposed about an empty, cylindrical interior. During operation, two phases of an RF voltage are applied sequentially in turn to each pole rod. In an axis of the rod system (i.e., an axis of the cylindrical interior), the two phases cancel out such that there is a DC potential that corresponds to the zero crossing of the RF voltage. The interior of the rod systems have diameters ranging from around three to twenty millimeters. The RF voltage of the rod system ranges between a few tens and a few hundreds of volts at frequencies of around one to ten megahertz. The pole rods of the rod systems are typically constructed from metal. The multipole RF fields between the pole rods form so-called “pseudopotentials”, which push the ions centripetally toward the axis. In the potential well of the pseudopotential, the ions can oscillate through the axis or about it. These oscillations are called “secular oscillations”. In quadrupole rod systems, the pseudopotential increases as the square of a distance from the axis. Therefore, the secular oscillations are harmonic oscillations.
In addition to multipole rod systems, there are also other types of RF ion guides such as, for example, ion funnels or other ion guides consisting of ring electrodes. The direction and shape of the RF fields are substantially different from those of the aforementioned rod systems.
Ion guides typically contain a damping gas which dampens the motions of the ions, particularly their radial oscillations in the pseudopotential well, so that the ions quickly collect in the axis of the ion guides in a string-shaped cloud. This process is called “collisional focusing”. The diameter of the string-shaped cloud is determined by (i) the equilibrium between the mass-dependent, (ii) centripetal force of the pseudopotential generated by the RF voltage and the centrifugally acting force of the space charge, and (iii) by the strength of the residual thermal motion. The damping gas may be provided from a vacuum-external ion source to be introduced together with the ions into the vacuum system. Alternatively, the dampening gas and the ions may be introduced separately.
Ions are transported in the ion guide via forward movement of the string-shaped cloud and partly by the space charge of the ions still being supplied by the ion source. If the damping gas originates from a vacuum-external ion source, for example an electrospray ion source, differential pumping systems with several pump chambers are typically used, through which the ions must be guided. This pumping system generates a continuous flow of gas molecules in the ion guides, which drive the ions through a series of ion guides in the direction of the ion analyzer. Notably, the term “ion guide” should be construed broadly. For example, an ion guide may include quadrupole filters which serve to select ions of a particular mass range for further analysis, and collision cells which serve to fragment ions.
In quadrupole filters, DC voltages are superimposed onto both phases of the RF voltage. As a result, only ions having a narrow mass range can pass through the filter on a stable path. In contrast, the remaining ions become radially unstable and thereafter are typically lost from the beam. Quadrupole filters are usually operated at very low vacuum pressures, for example below approximately 10−3 Pascal, such that no damping of the ion motions occurs in them. Ideally, no collisions should take place with molecules of residual gas. They therefore lack the onward transport of the ions by the streaming gas. Rather, the ions, which were injected with a given kinetic energy, fly through the mass filter via their inertia without being decelerated.
As stated above, ions are often generated at atmospheric pressures and transported in the vacuum through a series of pump chambers of a differential pumping system. Advantageously, an apertured diaphragm (“diaphragm”) may be used to create a relatively small pumping cross-section in the wall between two of the pump chambers. For example, the aperture of the diaphragm can be sized almost as small as the diameter of the string-shaped cloud. If the diaphragm is disposed in the wall between the pump chambers in a sealed (i.e., air-tight) fashion, the aperture in the diaphragm defines the sole pumping connection. As a result, less expensive, lower performance pumps can be used. In such a configuration, the ion guides extend right up to the diaphragm on both sides. Alternatively, the cross-section of the ion beam can be matched to the acceptance profile of an adjacent downstream ion guide by an apertured diaphragm if the ion cloud does not already have a sufficiently small diameter.
It is further possible to use one or more diaphragms to create an ion lens which matches the ion beam to the acceptance profile. However, such a matching to the acceptance profile must be carefully observed when the ions are injected into a quadrupole filter. Using an accelerating ion lens, the ions can be accelerated into the next ion guide in order to fragment the ions via, e.g., relatively high-energy collisions.
Typically, prior art diaphragms are constructed from metal, or at least superficially metalized, to prevent electrical charges from building up on the surfaces and giving rise to undesirable, temporally varying field distortions. However, the metallic conductivity of the apertured diaphragms may short-circuit the RF field at the end of the ion guides. These short circuits may weaken the guiding field in the vicinity of the diaphragms such that considerable ion losses occur in front of and behind the diaphragm. In particular, components of the pseudopotential, which axially oppose the direction of the ion flow, may cause perturbations (i.e., turbulence) in the ion flow in the leakage field.
U.S. Pat. No. 3,867,632 and its continuations U.S. Pat. Nos. 3,936,634, 3,937,954 and 4,013,887, collectively referred to as the “W. L. Fite Patents” and hereby incorporated by reference, teach that a volume having superimposed RF and DC fields may be shielded such that, in the interior of the volume, the RF field is maintained practically unchanged, whereas the DC field is completely shielded. To provide such DC shielding, a material is used that has relatively weak conductivity and a resistance far in excess of 105 Ohm×cm (“leaky dielectric”). In the W. L. Fite Patents, the shielding is configured in the form of a tubular channel connection piece through which the ions are introduced into an RF quadrupole filter while shielding the superimposed DC voltages. However, introduction of ions into an RF quadrupole rod system operating as a mass filter can be difficult and result in high ion losses due to a relatively small acceptance cross-section and acceptance angle.
U.S. Pat. No. 4,283,626 to M. W. Siegel (“Siegel Patent”), which is hereby incorporated by reference, discloses a tube constructed from a relatively high-resistance material that enables a region having a higher gas density to be formed in the interior of an RF multipole rod system. This region is formed to fragment the ions via gas collisions, for example, without losing the focusing effect of the RF field on entrained ions. Such collision cells usually have a gas drain to both sides because they are surrounded on either side by low pressure devices. These devices include a quadrupole filter disposed upstream of the ion flow to select the ions to be fragmented, and a mass analyzer disposed downstream of the ion flow. Therefore, it is advantageous for the fragment ions produced in the collision cell to be actively transported out of the collision cell by an electric field.
The Siegel Patent further discloses that a DC voltage gradient can be generated along the axis of the multipole rod system with the aid of such a tube. Different potentials are applied to the two ends of the tube such that a weak current through the tube generates a voltage drop. However, metal wires are used to apply the voltages and the end surfaces of the tube are made conductive via a metallic paint which causes the RF field to break down in the vicinity of the supply wires and the tube ends. Therefore, there is a need in the art to supply voltages without interfering with the RF fields.